Numerous operations are known, in particular in mechanical engineering, in which a tool may be replaced by a focused laser beam. As an example there may be cited various machining operations, such as stamping or boring, assembly operations, such as welding, or surface heat treatment. In all these operations the laser beam is focused by a suitable optical system, such as a lens or a curved mirror; it thus forms a focusing cone whose apex, located in the focal plane of the optical system used, has the maximum light power density of the beam. For reasons of energy saving and operational precision, this maximum power point has to be positioned as accurately as possible with respect to the treated part; for example, in the case of surface treatment the focal point is usually brought into the plane tangential to the surface of the part at the point where the intended operation is to be performed.
Although it is important that this point is positioned correctly with respect to the part to be treated, it is equally important that this relative position remains as constant as possible.
This condition is the more difficult to fulfill if the parts are moving or if they sometimes have somewhat irregular surfaces. It is, however, indispensable for ensuring uniform and reproducible treatment of the part. The constancy of the relative position is the more critical, the shorter the focal length of the optical system, i.e. the greater the aperture angle of the focusing cone.
By way of example, a parallel laser beam with a diameter of 20 mm and a wavelength of 10.6 .mu.m, focused by a lens whose focal length is 60 mm, has a diameter of 0.1 mm at the focus, with a depth of field of 0.03 mm.
The position of the focus of an optical system with respect to the active element of this system, for example a lens, is generally known. In order to maintain this focus in the surface plane of the part, it is necessary to ensure and maintain a constant distance between the surface plane of the part and any point of the active element of the optical system. In the case of a lens, this point may in particular be the point at which the axis of the laser beam passes through the median plane of the lens.